The present invention relates to a clamp for measuring an electrical current flowing in conductors.
Clamps of this type generally comprise two jaws interconnected by a hinge and each provided with a segment of an annular magnetic circuit, each segment having a proximal end and a distal end relative to the hinge. The jaws are hinged to move between a closed position in which the distal and proximal ends of the segments are respectively in contact, and an open position in which the distal and proximal ends are spaced apart.
When the clamp is closed around a live conductor, the electrical current carried by the conductor induces magnetic flux in the annular magnetic circuit that has been closed in this way. A measurement member associated with the clamp measures this magnetic flux which is proportional to the current and it calculates the value of the current.
To avoid magnetic flux losses and thus obtain measurement that is accurate, it is necessary for good quality magnetic contact to be established via the proximal ends and via the distal ends of the segments when the clamp is closed. At least one segment is thus mounted to rock about a rocker axis parallel to the hinge axis so that the segments can be applied properly against each other at their distal and proximal ends.
In an installation comprising a plurality of conductors, and in particular bare conductors, the operator handling the clamp must take care not to set up a short circuit between two adjacent conductors by contact between them and the magnetic circuit. To limit this risk, the segments are covered in an insulating coating with the exception of the distal and proximal ends. Nevertheless, with clamps of the above-described type, there exists a risk that the distal ends of an open clamp will come into contact with two adjacent conductors, and because the segments are movable relative to the jaws, the proximal ends of the segments will come into contact, thereby establishing a short circuit between two conductors. Such short circuiting is dangerous for the operator and runs the risk of damaging the installation.
It has thus been proposed to surround one of the distal ends with a protective cover, with the distal end being set back therein. The cover enables the distal end fitted in this way to make contact with the distal end of the other segment, but it prevents the distal end from coming into contact with a conductor, such that even if the proximal ends are in contact there is no short circuit between conductors. Nevertheless, the cover increases the size of the distal end which makes it difficult to insert it between the conductors that are to be measured. In addition, such a cover is very exposed to impacts that might damage it, thereby reducing or eliminating protection. Furthermore, various particles can accumulate in the setback formed by the cover, thereby preventing good contact being established with the other distal end. In addition, the cover makes it impossible to verify by eye the that contact has been properly established between the distal ends.
An object of the invention is to provide a clamp having means for preventing a short circuit being established between two adjacent conductors without using a cover on the distal ends of the magnetic circuit segments.
In order to achieve this object, the invention provides a measuring clamp for measuring an electric current carried by conductors, the clamp comprising first and second jaws connected together by a hinge and provided with respective first and second segments of an annular magnetic circuit, each segment having a proximal end and a distal end relative to the hinge, and at least the first segment being mounted to rock about a rocker axis parallel to a hinge axis, the jaws being movable about the hinge axis between a closed position in which the distal ends and the proximal ends of the segments are respectively in contact, and an open position in which the distal ends and the proximal ends are spaced apart, the clamp including spacer means for spacing apart the proximal ends, which means are active at least while the distal ends are spaced apart by a distance that is greater than or equal to a predetermined safety distance.
The safety distance is determined so as to correspond to the minimum distance separating two conductors, given the voltage to which they are raised. Thus, the proximal ends are held apart from each other at least so long as there remains any risk of the distal ends coming into contact with two adjacent conductors. The magnetic circuit is thus kept open at its proximal ends, and the risk of a short circuit being established is eliminated.
In an advantageous version of the invention, the spacer means comprise a moving member mounted to slide between an active position in which the moving member has a cam-forming portion co-operating at least with the proximal end of the first segment to space it apart from the proximal end of the second segment, and an inactive position in which the moving member releases relative movement between the segments.
In a first embodiment, the moving member in the inactive position is associated with a stop for locking the jaws closer than an intermediate open position in which the distal ends are spaced apart by a distance equal to the safety distance, and the moving member in the active position escaping from this stop. Any opening of the clamp beyond the safety distance thus requires the moving member to be pushed into the active position.
In a second embodiment, the moving member extends between at least one cam surface carried by one jaw and an adjacent projection at the proximal end of a magnetic circuit segment mounted to rock on the corresponding jaw, the cam-forming portion of the moving member extending in register with the corresponding projection of the magnetic circuit segment. Thus, opening the clamp automatically activates the spacer means.